1. Field of the Invention
The present invention is a multilayer substrate comprising a layer of diamond and a composite layer comprising particles of diamond and silicon carbide, and, optionally, particles of silicon and a method of making the multilayer substrate.
2. Description of Related Art
Diamond is the hardest material known, having a Mohs Hardness of 10, which makes diamond most useful for applications of cutting, machining, drilling, milling, etc. Diamond is also the most thermally conductive material known, having a thermal conductivity up to 2000 to 2200 watts per meter per degree Kelvin (K), which makes it highly desirable for applications in thermal management under demanding conditions. Diamond also has an extremely low coefficient of friction, which makes it a versatile material for uses such as brakes.
Diamond is also an excellent optical material for transmitting microwave, infrared, visible, and other ultraviolet electromagnetic waves. Diamond also has high stability when used as detector for high fluence nuclear radiation. In addition, diamond is also a highly inert material in chemical environments that might involve strong acids, strong bases, strong oxidizing agents, or strong reducing agents, even at elevated temperatures or at cryogenic conditions. Furthermore, diamond is one of high refractive index materials, which leads to its popular and most premium values in jewelry industries.
Information regarding diamond can be found in the following references, (1) “Properties, Growth and Applications of Diamond”, Edited by M. H. Nazare and A. J. Neves, 2001, published by The Institute of Electrical Engineers; (2) “Diamond Films Handbook”, edited by Jes Asmussen and D. K. Reinhard, 2002, published by Marcel Dekker; and (3) “Diamond Films, Chemical Vapor Deposition for Oriented and Heteroepitaxial Growth”, Edited by Koji Kobashi, 2005, published by Elsevier.
Though diamond is one of the most versatile and most premium materials, its availability is very limited in nature. Moreover, diamond mined from the earth is typically of single crystal whose geometrical dimensions are very limited in size, most of the time, too small for industrial uses that require large dimensions. Many times, diamond formed in nature also contains impurities and crystal defects. The diamond crystal that is relatively large in crystal size, relatively pure in chemical contents, and relatively perfect without crystal defects is very expensive, often times, priceless.
Synthetic diamond is known to be produced industrially in chemical reactors under extremely high pressures and extremely high temperatures, called a high temperature and high pressure (HTHP) process. Due to harsh growth conditions, the reactor size is often limited, as are the dimensions of diamonds produced from the HTHP process, not to mention its associated high cost in process, equipment, and safety. Often times, the HTHP process produces diamond having a yellow tint due to the incorporation of catalytic impurities into the diamond lattice.
Industrially, single crystal diamond can also be grown in reactors in a process called chemical vapor deposition (CVD), where suitable growth conditions can be achieved by microwave-enhanced plasma, tungsten hot-filament, DC-Jet plasma, laser-induced plasma, acetylene-torch, etc. It is well known in art that CVD growth processes can also successfully grow polycrystalline diamond thin films on different substrates and/or free standing diamond thick films, though it is challenging to obtain low stress films or non-cracking diamond of significance in size. However, the CVD process typically produces diamond pieces that can be significantly greater than the diameter of single crystal diamond from nature or grown from the HTHP process. Nevertheless, the growth rate of diamond in CVD process, or any diamond growth process is generally slow, in a range from a growth rate of less than 1 micron/hr to a growth rate of no more than about 10 to 20 microns/hr, though some claim to be able to grow single crystal at a higher growth rate, but with many defects.
Growing a thick film of diamond on a substrate to form a composite of a layer of diamond film on a layer of the substrate is challenging due to the presence of extreme stress resulting from very different physical and chemical properties between the diamond and the substrate on which the diamond grows, such as, thermal conductivity, electrical conductivity, coefficient of thermal expansion, Young's modulus, etc. Chemically, the material of the substrate needs to be able to form carbide, a necessary bonding between atoms of chemical element of the substrate and atoms of carbon, which creates an interface that has some kind of affinity for other carbon bond containing materials to attach to, at least, by surface physical interactions such as van der Waal's forces, so that diamond crystals can seed on, which also bridges some chemical differences between diamond and the substrate. Silicon, tungsten, molybdenum, silicon carbide, tantalum, niobium, etc., are the carbide former, for which carbon from carbide provides some anchoring mechanism for diamond to stick to. This may be the reason that prior art has been able to demonstrate the some successes in depositing a diamond layer on this type of substrate under certain growth conditions. However, the interactions between diamond and the substrate may be in the physical interaction level by van der Waals' forces, or at a chemical level with some carbon-carbon bonding, at most, not even to mention that there is a crystal lattice mismatch between diamond lattices and metal carbide lattices, for which carbon-carbon bonds between diamond carbon and substrate carbon would be imperfect, creating stress defects for failures at the interface.
With respect to the physical properties, diamond and the substrate materials are also very different. For example, silicon has a thermal conductivity of 149 W/m-K, tungsten 173 W/m−K, molybdenum 138 W/m−K, tantalum 57.5 W/m−K, and niobium 53.7 W/m−K, etc., while diamond has a thermal conductivity of 2000 to 2200 W/m-K. Silicon has a coefficient of thermal expansion of 2.7×10−6/m/m-K, tungsten 4.6×10−6/m/m-K, molybdenum 4.8×10−6/m/m-K, tantalum 3.6×10−6/m/m-K, niobium 4.0×10−6/m/m-K, etc., while diamond only has a coefficient of thermal expansion of 1.0×10−6/m/m-K. Silicon has an electrical resistivity of 103 Ω-m, tungsten 52.8×10−9 Ω-m, molybdenum 53.4×10−9 Ω-m, tantalum 131×10−9 Ω-m, niobium 152×10−9 Ω-m, etc., while diamond has an electrical resistivity of 1011 Ω-m. Herein, when used as to represent a unit for a numerical value, “m”=meter and “K”=degree Kelvin.
The extreme differences in physical properties between diamond and such substrate materials create challenges to grow a thick diamond film or layer on these substrate materials without delamination, in addition to the intrinsic limitations resulting from chemical bonding of diamond and substrates, if there were any. It is envisioned that a diamond film CVD grown on these substrates is probably highly stressed prior to delamination from the substrate, which happens during deposition sometimes, and which happens after shutting down the reactions, sometimes. Even if a diamond film survives without delamination, this diamond film, most of times, a thin film, is still highly stressed, which is highly undesirable for various applications, since the film might delaminate while standing alone, or delaminate while being used for different purposes.
Prior art takes advantages of delamination behaviors of a diamond film on a substrate to separate the diamond film from a substrate and produce a free-standing diamond film, though difficult. The delamination process may involve vast amounts of stress. To this end, it is difficult to obtain a crack-free diamond thick film as a thick diamond film might shatter into many small pieces. When the diameter, or geometrical dimension, of a diamond film or layer increases, the problem of preserving the geometry without cracking, thick or thin, become even more challenging, or impossible. Sometimes a diamond film partially delaminates and leaves some substrate areas with some undelaminated diamond, which prevents reuse of this substrate in the next diamond film growth. Since diamond is the hardest material on earth, grinding off residual diamond film from a substrate is difficult, time-consuming and expensive. Even when surviving the delamination, a diamond thin film, even with small dimensions, is very fragile and difficult to handle, which makes it impossible to use a free-standing thin film of diamond for industrially practical applications.
In many practical applications, a unique and intact composite of a diamond layer on a substrate layer, having a minimum or reduced stress between the diamond layer and the substrate layer, without cracking or shuttering or without risk of being cracked or shattered while standing or while in use, is highly desirable. Sometimes, a thin diamond layer is desirable when supported on an inexpensive substrate, with minimum or reduced stress enabling its successful use in harsh conditions, found in machining, drilling, cutting, milling, etc. Sometimes, a composite of a thick diamond layer on a substrate layer of large geometric dimensions, having a minimum stress or reduced stress, crack-free or having a minimum level of crack, is desirable for applications like optical mirrors, thermal management, friction control, mechanical uses like drilling, machining, cutting, milling, etc.